The present invention relates in general to the field of burners.
It applies in particular to burners that use a pulverulent fuel.
It relates for example but nonlimitingly to burners for rotating (or rotary) kilns, such as cement kilns or lime kilns.
There are two main types of such burners for rotary kilns, those referred to as “direct-fired” and those referred to as “indirect-fired”.
In burners for rotary kilns referred to as “direct-fired”, the pulverulent fuel is transported by the air, referred to as the primary transport air, that was used in the process of drying/grinding the pulverulent fuel, in a longitudinal main pipe.
The rest of the air, referred to as the secondary air, surrounds the main pipe and allows combustion. It is generally supplied in the form of the hot air resulting from the processes of cooling the material fired in the kiln, and which may be at a temperature of 500 to 1100° C.
The longitudinal main pipe may comprise a concentric internal pipe, equipped with pipes for conveying other fuels, it being possible for the other fuels to be a liquid such as oil, a gas, or a solid for igniting the primary transport air and the secondary air.
In burners for rotary kilns referred to as “indirect-fired”, the air used for the process of drying/grinding the pulverulent fuel is filtered then discharged (or recirculated to elsewhere) without participating in the carriage of the pulverulent fuel to the burner. The pulverulent fuel is transported with a quantity of transport air which is far lower than in burners for rotary kilns what are referred to as “direct-fired”.
The primary transport air with pulverulent fuel is transported to the burner in a secondary pipe concentric with and on the inside of a longitudinal main pipe.
The longitudinal main pipe may on the inside of the secondary pipe comprise a concentric internal pipe equipped with pipes for conveying other fuels, it being possible for the other fuels to be a liquid such as oil, a gas, or a solid for igniting the primary transport air and the secondary air.
A primary air without pulverulent fuel and which corresponds to fresh air, emerges at very high speed in pipes situated concentrically to the secondary pipe and to the main pipe, on the inside and/or on the outside of the main pipe.
The flows of primary air in these pipes may be axial or swirling (peripheral) on account of the presence of swirl-inducing means.
The rest of the air, referred to as secondary air, surrounds the longitudinal main pipe and allows combustion. It is generally supplied in the form of hot air originating from the processes of cooling the material fired in the kiln, and may be at a temperature of 500 to 1100° C.
The high speed and axial and/or swirling flows of the primary air ensure mixing between the primary transport air and the secondary air, in order to achieve combustion.
At its center, the longitudinal main pipe comprises an internal pipe equipped with: a flame stabilizer and fuel-conveying pipes, it being possible for the fuel to be a liquid such as oil, a gas, or a solid allowing the primary transport air and the secondary air to be ignited.
There is a need to achieve more rapid ignition of a “direct-fired” burner and an “indirect-fired” burner with a suitable concentration of the pulverulent fuel so as to achieve better volatilization of the pulverulent fuel and thereby make it easier to ignite. This rapid ignition of the pulverulent fuel has numerous advantages such as the reduction of oxides of nitrogen through the effect of the staging of the combustion, improved combustion of alternative fuels, and improved thermal profile within the kiln.